A gas turbine engine, per FAA regulations, should be capable of ingesting foreign objects (e.g., birds in flight) while allowing for continued operation, or safe and orderly shutdown of the engine. Further, blades in the gas turbine engine should be resistant to cracking due to nicks and/or dents caused by small debris such as sand and/or rain. To prevent damage on account of such small debris and foreign object ingestion, specifically, materials such as titanium alloys and fiber composites are typically used to construct the fan and/or compressor blades in gas turbine engines. Gas turbine engines with titanium fan blades, as well as certain reinforced fiber composite fan blades with adhesively bonded metallic leading edge sheaths, are commonly used to meet such damage-prevention criteria.
While titanium blades may be relatively strong, they also may be relatively heavy and expensive to manufacture. Further, composite blades may offer sufficient strength and be significantly lighter than titanium blades; however, composite blades are expensive to manufacture. Also, due to their relatively low strain tolerance, composite blades may require a greater thickness than otherwise equivalent metal blades to meet requirements for ingestion of foreign objects. Increases in fan blade thickness may be accompanied by decreases in fan efficiency. This decrease in efficiency may offset a portion of the efficiency gains from the decrease in weight achieved by using a composite blade.
As another alternative, fan and/or compressor blades made from aluminum or aluminum alloy may result in significant decreases in weight over titanium blades and may be less expensive to manufacture than composite blades. However, aluminum and/or aluminum alloy blades may be softer and lower in strength than titanium and/or composite blades. Aluminum blades may also be susceptible to erosion and corrosion, and therefore require coatings. In some designs, a leading edge sheath made of titanium and/or nickel can give the aluminum blade added protection without significantly increasing the weight.
While each is effective in certain aspects, modern engine design often requires larger and larger blades. The foregoing weight, strength, and foreign object resistance issues are only exasperated in such larger blades. Accordingly, it can be seen that further improvements in blade design are needed.